Polyethylene is the most widely used commercial polymer and can be prepared by different processes. Polymerization in the presence of free-radical initiators at elevated pressures was the method first discovered to obtain polyethylene and continues to be of commercial relevance for the preparation of low density polyethylene (LDPE).
A common set-up of a plant for preparing low density polyethylene comprises a polymerization reactor and reaction components that may be pressurized by a combination of two compressors, a primary compressor and a secondary compressor. At the end of the polymerization sequence, a high-pressure polymerization unit may further include apparatuses like extruders and granulators for pelletizing the resulting polymer. Furthermore, such a polymerization unit generally may also comprise means for feeding monomers and comonomers, free-radical initiators, chain transfer agents and/or other substances at one or more positions into the polymerization reaction. A process and an apparatus for the manufacture of ethylene polymers and copolymers under high pressures are disclosed in WO 2007/018871 A1.
A characteristic of the radically initiated polymerization of ethylenically unsaturated monomers under high pressure is that the conversion of the monomers is often incomplete, as only about 10% to 50% of the dosed monomers are converted. The resulting reaction mixture may leave the reactor through a pressure control valve, often designated as a let-down valve, and may then be separated into polymeric and gaseous components with the unreacted monomers being recycled. To avoid unnecessary decompression and compression steps, the separation into polymeric and gaseous components is usually carried out in two stages. The monomer-polymer mixture leaving the reactor is transferred to a first separating vessel, frequently called a high-pressure product separator, in which the separation into polymeric and gaseous components is carried out at a pressure that allows recycling of the ethylene and comonomers separated from the monomer-polymer mixture to the reaction mixture at a position between the primary compressor and the secondary compressor. At the first separation vessel operating conditions, the polymeric components within the separating vessel are in liquid state. The liquid phase obtained in the first separating vessel is transferred to a second separation vessel, frequently called a low-pressure product separator, in which further separation into polymeric and gaseous components takes place at lower pressure. The ethylene and comonomers separated from the mixture in the second separation vessel are fed to the primary compressor where they are compressed to the pressure of the fresh ethylene feed, combined with the fresh ethylene feed and the joined streams are further pressurized to the pressure of the high-pressure gas recycle stream.
The recycling of unreacted monomers to the inlet of the reactor and conducting the recycle at high pressures, which may reduce the need for re-compressing, are measures that may improve the economics of high-pressure polymerization processes. Nonetheless, carrying out the recycling requires considerable efforts commensurate with the amount of monomer converted to polymer per pass of the reactor. Consequently, there is a demand for conducting high-pressure polymerizations for maximizing the conversion of monomers per pass of the reactor to produce the targeted low density polyethylene grades. However, the possibilities for influencing the conversion of monomers by changing polymerization conditions are limited since the properties and the structure of the resulting ethylene homopolymers or copolymers, such as molecular weight, molecular weight distribution and the amount of short- and long-chain branching, depend strongly on the reaction parameters.
Furthermore, various articles and applications of low density polyethylenes such as blown films often require a narrow molecular weight distribution of the low density polyethylene for achieving a good balance of optical and mechanical properties. Accordingly, there is a demand for low density polyethylenes with narrow molecular weight distributions in a high-pressure polymerization process.
Hence, there is a need to overcome the disadvantages of the prior art and to provide a process which makes it possible to polymerize or copolymerize ethylenically unsaturated monomers in a tubular reactor with a high conversion of monomers to polymer per pass of the reactor. Furthermore, the process should allow for the preparation of low density polyethylenes with a high conversion rate per pass of the reactor without introducing detrimental effects in the article forming processes or on properties of the produced low density polyethylenes.